The invention relates to electrically switchable storage arrays. More particularly, the invention relates to switchable permanent or read-only storage arrays with coupling elements provided at the crosspoints of a matrix arrangement. The coupling elements are functionally personalized to selectively provide electrically conductive connections. Permanent storage arrays such as read only memories (ROMs) and programmable logic arrays (PLAs) are used in the computer field for storing fixed data permanently, i.e. nondestructively. PLAs have been discussed as increasingly interesting embodiments of permanent storage matrices, which are described in general in the book "MOS/LSI Design and Application" by W. Carr and J. Mize, McGraw Hill 1972, pages 229 to 258.
Such PLAs composed of ROMs (AND/OR arrays) permit the direct implementation of (minimized) combinatorial logic in the matrix arrangements of regular array structures. Compared with the irregular structure of logic systems, such permanent storage arrays are advantageously used as unilateral correlators, making them increasingly attractive as replacements for combinatorial logic, as storages for tables, constants and programs, as hard wired multipliers, and for code generation/conversion, etc.
In ROMs as well as PLAs constructed with ROMs, personalization of the respective function(s) is generally accomplished during manufacture and is then fixed once and for all. In order to eliminate the resultant inflexibility various alternatives have been suggested. One alternative is a re-programmable read-only storage, as described in U.S. Pat. No. 4,041,459. Although re-programmable, it has a number of disadvantages: as coupling elements or storage cells, respectively MI.sub.1 I.sub.2 S-FETs, i.e. FETs with double layer gate dielectric are required which compared with MOS field effect transistors (FET) are much more complex from a technological point of view. Furthermore, reprogramming requires relatively high voltage pulses in the order of 30 V. Such reprogrammable read-only storages are far too slow for many uses with respect to changeability.
Programmable read-only memories having faster electrical switching speed are also known. For example, U.S. Pat. No. 3,987,286 describes an arrangement where, through time-controlled release or inhibition of logic circuits of the matrix arrangements, different logic functions can be executed in one and the same PLA. A disadvantage of this arrangement is that personalization data must be stored in additionally required shift register elements. Another known type of multiple personalization with specific coupling or logic elements which e.g. can be set to four possible states, is described in IBM TDB Vol. 17, No. 3, August 1974, pages 811/812. Due to the specific components required, and the relative slowness involved, these solutions still need improvement.
Another known type of such read-only memories and particularly PLAs share unused logic areas for further functions, as described in IBM TDB Vol. 20, No. 10, March 1978, pages 4016 to 4018, or provide a multiple use of such areas, as described in U.S. Pat. No. 4,084,152. However, considerable restrictions remain with respect to the degree of freedom left for the designer of complex logic networks. Most of the high degree of flexibility and easy adaptability usually desired from such regular array structures is lost.
Finally, in connection with the structure of electrically alterable permanent storage arrays, memories having MNOS transistor cells or a transistor with "floating" gate in series with one respective associated separating transistor of the conventional MOS type have been described, as for example in GE-OS 28 44 955 and GE-OS 29 37 337. Similarly, as in the above described storage arrays with electrical re-programming means, to which group these storage arrays also belong, there is no quick switching between several functions.
Accordingly, it is an object of the invention to provide an improved permanent storage array as they can be used for PLAs, to the effect of achieving higher functional densities. It is another object of this invention to provide quick electrical switching between different functions in a permanent storage, without a requirement for high voltages or special components for re-programming.